The following relates to the optoelectronic arts. It finds particular application in testing angular distribution of light generated by light emitting devices such as phosphor-coated light emitting diode (LED) based devices, or light processed by optical components such as phosphor coated domes, lenses, frosted diffusion domes, and so forth. The following will find more general application in conjunction with testing spatial distributions of light generated by light sources or processed by optical components.
A characteristic of a wide-angle light source or optical component that is sometimes of interest is the angular distribution of light generated by the light source or processed by the optical component. For example, a conventional light emitting diode chip typically emits in a generally Lambertian pattern, in which the intensity at the sides is substantially decreased compared with intensity directly in front of the chip. In some applications a more hemispherically uniform output is desired. One approach for achieving such an output is disclosed in U.S. Pat. No. 7,224,000 which discloses, in some illustrated embodiments, a light source including one or more blue, violet, or ultraviolet light emitting diode chips mounted on a circuit board and covered by a phosphor-coated dome. The phosphor converts the direct chip radiation to a desired output, such as white light. Since this conversion occurs across the surface of the dome, a more hemispherically uniform light output is generated by the light source. Other approaches for achieving more hemispherically uniform light output include epoxy encapsulation, use of microlenses, and so forth. Moreover, in some applications a Lambertian or other angular distribution may be desired. Thus, it is desirable to have a rapid and effective way to characterize the angular distribution.
In one approach, a camera acquires images of the operating light source, or of the optical component under test in its usual operative arrangement. By acquiring images at several different viewing angles, the angular distribution can be extrapolated. This approach entails substantial post-acquisition image analysis to extrapolate the three-dimensional angular distribution from the two-dimensional acquired images. Such extrapolation, light diffraction in the imaging optics, or other factors, can adversely affect the extrapolated angular distribution.
A more elaborate version of this approach is employed by the SIG-300™ Source Imaging Goniometer (available from Radiant Imaging, Duvall Wash.). This apparatus captures uses a goniometer configuration to acquire images and flux measurements from thousands of angular viewpoints around the source. Other goniometer-based apparatuses employ a single point detector. Such apparatuses can provide more data from which to extrapolate the angular distribution. However, the goniometer is a complex mechanical device which mechanically changes the angular position of the camera or detector prior to each measurement. As a result, such apparatuses are relatively slow, and can be subject to mechanical failures.